1. Field of the Invention
The invention relates to an MR method for two-dimensional or three-dimensional imaging of an examination zone, in which a set of raw data is repeatedly measured and an image of the examination zone can be reconstructed from each set, in which the raw data of a set is acquired with different measurement parameters, and in which different sets contain raw data acquired with the same measurement parameters.
2. Description of the Related Art
Measurement parameters in this respect are magnitude and/or direction of the magnetic gradient fields acting, or having acted, on the examination zone as from the excitation of the nuclear magnetization in the examination zone by one or more RF pulses until acquisition of the MR signal. The temporal resolution of an MR image produced by an MR method (MR=Magnetic Resonance) is determined by the measurement period within which the set of raw data required for an MR image can be acquired. Dynamic processes in the examination zone, for example the propagation of a contrast medium, which occur within a period of time which is shorter than the measurement period can also influence an MR image, but the known method of the kind set forth cannot provide an indication as to when these processes have occurred during the measurement period. A similar limitation in respect of spatial resolution occurs in the case of slice images of a moving patient.
It is to be noted that from U.S. Pat. No. 4,710,717 an MR method is known for the imaging of periodically pulsating objects, notably the human heart, in which several sets of raw data are measured, the position in time relative to the cardiac cycle during which they are acquired being assigned to the raw data by way of an ECG signal. The sets of raw data, however, are not measured successively in time, but quasi-simultaneously in that during each cardiac cycle several MR sequences with identical magnetic gradient fields act on the examination zone. The phase encoding gradients are changed from one cardiac cycle to another, so that the acquisition of the raw data sets is completed after a number of cardiac cycles which is dependent on the spatial resolution. The reconstruction of the examination zone in a given phase of the cardiac cycle utilizes the raw data measured during this phase in the individual cardiac cycles. If no raw data has been acquired during the relevant phase in a cardiac cycle, the desired raw data is calculated from the raw data of these sequences measured directly before and directly after the desired phase in the relevant cardiac cycle.
The MR image reconstructed by means of this data thus contains data from each cardiac cycle, so that improvement of the temporal resolution is not possible. The possibility of imaging individual phases of the cardiac action with a comparatively small amount of motional unsharpness is based on the fact that the nuclear magnetization distribution in the examination zone is periodically recurrent. When this is not the case, for example in the event of an examination by means of a contrast medium (during the phase in which the contrast medium flows into the examination zone), the known method fails.